One could say modern sex scandals in politics come to mind, think perhaps of famously Bill Clinton.

You'll find older examples, too. However, often they are more up for debate due to the context. Case in point, the case of the French king Philip I (1060-1108), who was put under Interdict by the pope (multiple times) for taking another (married) woman as his own. Whilst Philip repudiated his former wife, the Reform Papacy retaliated on moral grounds. The whole affair lasted years, and while Philip made it seem as if he broke of his second marriage, he stayed with his new woman. It went so far that the former husband (the Count of Anjou) of his new wife retracted his allegiance to the house of Capet & placed Anjou under the suzerainity of the Papacy... Clerical writers on the side of the Papacy couched this affair in terms of weakness of the flesh.

The reality was, however, more prosaic.

Philip's former wife was eventually barren, and with only one male heir, the king had his dynastic duty to consider and ensure more offspring. The feelings he might have held for his new woman we will never truly know. It is important to consider that in early medieval Europe, marriage as a singular concept was not yet established, and various forms of matching existed. Basically, noble and clerical values clashed as both sides were in the process of establishing social norms that in this regard, conflicted.

So the Pope and his cronies would say it was weakness of the flesh, but king Philip arguably had other things on his mind.